Dimensionality engineering of metal halide perovskites

Structure-Related Evolution of Magnetic Order in Anisidinium Tetrachlorocuprates(II)

Tetrachlorocuprate(II) hybrids of the three anisidine isomers (ortho-, meta-, and para-, or 2-, 3-, and 4-methoxyaniline, respectively) were prepared and studied in the solid state via X-ray diffraction and magnetization measurements. Depending on the position of the methoxy group of the organic cation, and subsequently, the overall cation geometry, a layered, defective layered, and the structure comprising discrete tetrachlorocuprate(II) units were obtained for the para-, meta-, and ortho-anisidinium hybrids, respectively. In the case of layered and defective layered structures, this affords quasi-2D-layered magnets, demonstrating a complex interplay of strong and weak magnetic interactions that lead to the long-range ferromagnetic (FM) order. In the case of the structure with discrete CuCl₄^2- ions, a peculiar antiferromagnetic (AFM) behavior was revealed. The structural and electronic origins of magnetism are discussed in detail. To supplement it, the method for calculation of dimensionality of the inorganic framework as a function of interaction length was developed. The same was used to discriminate between n-dimensional and "almost" n-dimensional frameworks, to estimate the organic cation geometry limits for layered halometallates, and to provide additional reasoning behind the observed relation between cation geometry and framework dimensionality, as well as their relation to differences in magnetic behavior.

Emerging Trends of Carbon-Based Quantum Dots: Nanoarchitectonics and Applications

Carbon-based quantum dots (QDs) have emerged as a fascinating class of advanced materials with a unique combination of optoelectronic, biocompatible, and catalytic characteristics, apt for a plethora of applications ranging from electronic to photoelectrochemical devices. Recent research works have established carbon-based QDs for those frontline applications through improvements in materials design, processing, and device stability. This review broadly presents the recent progress in the synthesis of carbon-based QDs, including carbon QDs, graphene QDs, graphitic carbon nitride QDs and their heterostructures, as well as their salient applications. The synthesis methods of carbon-based QDs are first introduced, followed by an extensive discussion of the dependence of the device performance on the intrinsic properties and nanostructures of carbon-based QDs, aiming to present the general strategies for device designing with optimal performance. Furthermore, diverse applications of carbon-based QDs are presented, with an emphasis on the relationship between band alignment, charge transfer, and performance improvement. Among the applications discussed in this review, much focus is given to photo and electrocatalytic, energy storage and conversion, and bioapplications, which pose a grand challenge for rational materials and device designs. Finally, a summary is presented, and existing challenges and future directions are elaborated.

Recent Advances on the Strategies to Stabilize the α-Phase of Formamidinium Based Perovskite Materials

Perovskite solar cells (PSC) are considered promising next generation photovoltaic devices due to their low cost and high-power conversion efficiency (PCE). The perovskite material in the photovoltaic devices plays the fundamental role for the unique performances of PSC. Formamidinium based perovskite materials have become a hot-topic for research due to their excellent characteristics, such as a lower band gap (1.48 V), broader light absorption, and better thermal stability compared to methylammonium based perovskite materials. There are four phases of perovskite materials, named the cubic α-phase, tetragonal β-phase, orthorhombic γ-phase, and δ-phase (yellow). Many research focus on the transition of α-phase and δ-phase. α-Phase FA-based perovskite is very useful for photovoltaic application. However, the phase stability of α-phase FA-based perovskite materials is quite poor. It transforms into its useless δ-phase at room temperature. This instability will lead the degradation of PCE and the other optoelectronic properties. For the practical application of PSC, it is urgent to understand more about the mechanism of this transformation and boost the stability of α-Phase FA-based perovskite materials. This review describes the strategies developed in the past several years, such as mixed cations, anion exchange, dimensions controlling, and surface engineering. These discussions present a perspective on the stability of α-phase of FA-based perovskite materials and the coming challenges in this field.

Synthesis, characterization and optoelectronic properties of 2D hybrid RPbX4 semiconductors based on an isomer mixture of hexanediamine-based dications

Three new hybrid two-dimensional (2D) organic–inorganic semiconductors are presented, which contain lead halides and a mixture of hexanediamine-based isomers in the stoichiometry [2,2,4(2,4,4)-trimethyl-1,6-hexanediamine]PbX4 (X = I, Br, Cl). These hexanediamine derivatives, with attached methyl groups at the carbon backbone of both isomers, determine the packing of the organic layers between the inorganic 2D sheets, while the optical absorption and photoluminescence spectra reveal excitonic peaks at T = 77 K and room temperature. The as-synthesized semiconductors were stored for three years in the dark and under low humidity and were examined again and the results were compared to those of the fresh materials. The chloride analogue, after the three year storage, displays white-like luminescence. The use of non-equivalent isomer and racemic mixtures in the organic component to form hybrid organic–inorganic semiconductors is an efficient method to alter the properties of 2D perovskites by tuning the isomers’ chemical functionalities. Finally, a comparison of the observed excitonic absorption and photoluminescence signals to that of analogous 2D compounds is discussed.

Realization of vis-NIR Dual-Modal Circularly Polarized Light Detection in Chiral Perovskite Bulk Crystals

Circularly polarized light (CPL)-sensitive direct detection is attracting increasing attention owing to its various optical technology applications and ultracompact device structures. However, current CPL-sensitive direct detection mainly focuses on a single mode, whereas the visible-near-infrared (vis-NIR) dual-modal detection, which is important for improving device sensitivity and night-vision performance, still remains to be explored. Here, for the first time, the vis-NIR dual-modal CPL-sensitive direct detection is presented in bulk single crystals of two-dimensional chiral perovskite (R-BPEA)₂PbI₄ (R-BPEA = (R)-1-(4-bromophenyl)ethylammonium). Benefiting from the strong light-matter interaction of the layered structure, (R-BPEA)₂PbI₄ shows a two-photon absorption (TPA) coefficient of up to 55 cm/MW, which almost falls around the highest value of 2D hybrid perovskites. Notably, (R-BPEA)₂PbI₄ exhibits a high vis-NIR dual-modal CPL-sensitive direct detecting performance under both visible light (520 nm) and NIR light (800 nm), with the on/off ratios of current higher than 10³, and the anisotropy factors for photocurrent higher than 0.1. This work will shed light on the design of new chiral semiconductors with a large TPA coefficient and promote their applications in vis-NIR dual-modal CPL-sensitive direct detection.

High-sensitivity and wide-temperature-range dual-mode optical thermometry under dual-wavelength excitation in a novel double perovskite tellurate oxide

Novel double perovskite SrLaLiTeO₆ (abbreviated as SLLT):Mn⁴⁺,Dy³⁺ phosphors synthesized using a solid-state reaction strategy exhibit distinct dual-emission of Mn⁴⁺ and Dy³⁺. High-sensitivity and wide-temperature-range dual-mode optical thermometry was exploited taking advantage of the diverse thermal quenching between Mn⁴⁺ and Dy³⁺ and the decay lifetime of Mn⁴⁺. The thermometric properties in the range of 298-673 K were investigated by utilizing the fluorescence intensity ratio (FIR) of Dy³⁺ (⁴F_9/2→⁶H_13/2)/Mn⁴⁺ (²E_g→⁴A_2g) and the Mn⁴⁺ (²E_g→⁴A_2g) lifetime under 351 nm and 453 nm excitation, respectively. The maximum relative sensitivities (S_R) of the resultant SLLT:1.2%Mn⁴⁺,7%Dy³⁺ phosphor under 351 nm and 453 nm excitation employing the FIR technology were determined to be 1.60% K^-1 at 673 K and 1.44% K^-1 at 673 K, respectively. Additionally, the maximum S_R values based on the lifetime-mode were 1.59% K^-1 at 673 K and 2.18% K^-1 at 673 K, respectively. It is noteworthy that the S_R values can be manipulated by different excitation wavelengths and multi-modal optical thermometry. These results suggest that the SLLT:Mn⁴⁺,Dy³⁺ phosphor has prospective potential in optical thermometry and provide conducive guidance for designing high-sensitivity multi-modal optical thermometers.

One-Step Synthesis of SnI2·(DMSO)x Adducts for High-Performance Tin Perovskite Solar Cells

Contemporary thin-film photovoltaic (PV) materials contain elements that are scarce (CIGS) or regulated (CdTe and lead-based perovskites), a fact that may limit the widespread impact of these emerging PV technologies. Tin halide perovskites utilize materials less stringently regulated than the lead (Pb) employed in mainstream perovskite solar cells; however, even today's best tin-halide perovskite thin films suffer from limited carrier diffusion length and poor film morphology. We devised a synthetic route to enable in situ reaction between metallic Sn and I₂ in dimethyl sulfoxide (DMSO), a reaction that generates a highly coordinated SnI₂·(DMSO)_x adduct that is well-dispersed in the precursor solution. The adduct directs out-of-plane crystal orientation and achieves a more homogeneous structure in polycrystalline perovskite thin films. This approach improves the electron diffusion length of tin-halide perovskite to 290 ± 20 nm compared to 210 ± 20 nm in reference films. We fabricate tin-halide perovskite solar cells with a power conversion efficiency of 14.6% as certified in an independent lab. This represents a ∼20% increase compared to the previous best-performing certified tin-halide perovskite solar cells. The cells outperform prior earth-abundant and heavy-metal-free inorganic-active-layer-based thin-film solar cells such as those based on amorphous silicon, Cu₂ZnSn(S/Se)₄ , and Sb₂(S/Se)₃.

Material exploration via designing spatial arrangement of octahedral units: a case study of lead halide perovskites

Halide perovskites have attracted tremendous attention as semiconducting materials for various optoelectronic applications. The functional metal-halide octahedral units and their spatial arrangements play a key role in the optoelectronic properties of these materials. At present, most of the efforts for material exploration focus on substituting the constituent elements of functional octahedral units, whereas designing the spatial arrangement of the functional units has received relatively little consideration. In this work, via a global structure search based on density functional theory (DFT), we discovered a metastable three-dimensional honeycomb-like perovskite structure with the functional octahedral units arranged through mixed edge- and corner-sharing. We experimentally confirmed that the honeycomb-like perovskite structure can be stabilized by divalent molecular cations with suitable size and shape, such as 2,2'-bisimidazole (BIM). DFT calculations and experimental characterizations revealed that the honeycomb-like perovskite with the formula of BIMPb₂I₆, synthesized through a solution process, exhibits high electronic dimensionality, a direct allowed bandgap of 2.1 eV, small effective masses for both electrons and holes, and high optical absorption coefficients, which indicates a significant potential for optoelectronic applications. The employed combination of DFT and experimental study provides an exemplary approach to explore prospective optoelectronic semiconductors via spatially arranging functional units.